Side Chain Polyrotaxanes

Polymers are generally not discussed in this book. However, a polymer with rotaxane structures involving cyclodextrins in side chains 408 [36], pseudorotaxane superstructures [37], doubly twisted polyrotaxane [38] as well as infinite polyrotaxane network (Figure 8.2.5) [39] can be mentioned here. 

Different types of polyrotaxanes, depending on how the cyclic and the linear units are connected, have been conceived [6-8, 12], According to the location of the rotaxane unit, polyrotaxanes can be defined as main-chain systems, Types 4, 5, 6, 7, and 8 (rows one and two in Table 1), and side-chain systems, Types 9, 10, 11, and 12 (rows three and four in Table 1). In main-chain polyrotaxanes the rotaxane unit is part of the main chain. In side-chain polyrotaxanes, the rotaxane moiety is located in the side chain as a pendant group. Polyrotaxanes can also be classified as polypseudorotaxanes and true polyrotaxanes, depending on their thermal stability toward dethreading. Polypseudorotaxanes are those without BG (column one in Table 1), in which the rotaxane components can be disassociated from each other by external forces. True polyrotaxanes are those with BG at the chain ends or as in-chain units (column two in Table 1), in which the rotaxane units are thermally stable unless one or more covalent bonds is/are broken. 

The effective complexation of CD was also used for the preparation of side-chain polyrotaxanes [96-102], All of these side-chain systems are based on Method 4 or Method 5 (Figure 9) and were reported by Ritter and coworkers. For Method 4, the CD was threaded on to a small molecule bearing a functional group Y and with one end blocked, i.e., a hemirotaxane. The reaction of Y with X, a pendant functional group in a preformed polymer, gives thermally stable... 

A side-chain polyrotaxane of Type 10 was also obtained by the reaction of de-protonated poly(benzimidazole) with a long-chain bromide bearing a BG at one end in the presence of / -CD [103], Because the CD was threaded onto the side chain before its reaction with backbone to form a hemirotaxane, the preparation is essentially Method 4. 

More recently, Harada et al. applied the complexation process to side-chain systems via Method 6 (Figure 10), in which the guest sites were introduced as pendant groups and thereafter the CD was threaded onto them [104, 105]. Different types of hydrocarbon chain as pendant groups were studied for their compatibility with different CDs. As the cyclic was not blocked, the products can be viewed as side-chain poly(pseudo rotaxane)s of Type 9. Probably because of the rapid exchange process between threaded and unthreaded forms, the isolation of the solid-state polyrotaxane was not reported. 

Figure 14. Preparation of branched or cross-linked polymer via the formation of side-chain polyrotaxanes.

Swager and coworkers also applied the self-assembly process in side-chain systems [131, 132]. The bisphenylene crown ether was incorporated into a conjugated backbone, polyphenyleneacetylene 87. This polymer complexes with paraquat 88 to give a novel polyrotaxane structure (89). With a polythiophene backbone, a similar polyrotaxane was synthesized by the same approach. 

CD are very rigid molecules and polyrotaxanes derived from them are expected to be more rigid than the starting backbone. Poly(methyl methacrylate side chain rotaxane) 56 had a Tg 20°C higher than the backbone itself. The same observation was also seen in side chain poly(ether ketone) and poly(ether sulfone) systems [96-102]. 

It is necessary to point out that while various types of polyrotaxane have been conceived (Table 1), to date, only polyrotaxanes of Types 4, 5, 6, 7, 9, 10 and 11 have been reported. Polyrotaxanes of Types 8 and 12 are worth study this might provide more interesting information about the relationship between properties and structure. In addition to those discussed so far, other potential preparation approaches have also been conceived but have not been applied. These methods are simply summarized and demonstrated via those for the side-chain polyrotaxanes of Type 10 (Figure 18). They are (i) chemical conversion, (ii) polymerization of rotaxane monomers, (iii) clipping (cyclization in the presence of preformed polymer), and (iv) grafting. The corresponding methods for other types of polyrotaxanes in Table 1 are analogous [6-8, 12]. 

Fig. 13 Various types of main chain and side chain polyrotaxanes blocking group (shaded circle) hollow circle (cyclic component) ellipses (cyclics threaded by linear species) [from Gong and Gibson (reproduced with permission from ref. 58)].

T. Takata, H. Kawasaki, N. Kihara, Y. Furusho, Synthesis of Side-Chain Polyrotaxane by Radical Polymerizations of Pseudorotaxane Monomers Consisting of Crown Ether Wheel and Acrylate Axle Bearing Bulky End-Cap and Ammonium Group , Macromolecules, 34, 5449 (2001)... 

Ritter et al. [147-155] have been studying side chain poiyrotaxanes. They synthesized side chain poiyrotaxanes by amide coupling of polymer-carrying carboxylic acid moieties with various semirotaxanes of methylated /l-CD(s) and an axle bearing an amine group at one end [147-154]. These works have been reviewed in an excellent review by Raymo and Stoddard [78]. Ritter et al. [155] reported recently a new type of side chain polyrotaxane. They polymerized inclusion complexes of di(meth)acrylates of butan-l,4-diol and hexan-l,6-diol with a-CD and with methylated /1-CD using a redox initiator system in aqueous media, and characterized the polyrotaxane structure by IR and glass-transition temperature measurements. 

Poly(benzimidazole) can be used as the reactive polymer for the preparation of side chain-type polyrotaxane via the alkylation on nitrogen. Osakada et al. showed that deprotonation of poly(benzimidazole) followed by N-alkylation with alkyl bromide-terminated pseudorotaxane consisting of 2,3,6-trimethyl-/1-cyclodextrin gave a novel side chain-type polyrotaxane (Scheme 38) [206]. Poly(benzimidazole) having a long -hydroxyalkyl group as the side chain can form an inclusion complex with 2,3,6-trimethyl-/l-cy-... 

Polymerization of pseudorotaxane is also a very effective method to prepare side chain-type polyrotaxanes in the case of cyclodextrin wheel. Ritter et al. claimed that the radical polymerization of pseudorotaxane consisting of 2,6-dimethyl-/ -cyclodextrin and alkyl chain with the acrylamide group and the big polycycloalkane group (cholesterol) at the both termini gave the... 

Buschmann et al. were the first to prepare a polyrotaxane containing cucurbituril by cleverly adapting interfacial polymerization conditions to the requirements of the water-soluble pseudorotaxane monomer. Polymers in which cucurbituril has been threaded onto the side chain in a pseudorotaxane fashion have been reported by Kim et al. Cucurbituril has also been integrated into a branched polymer structure. Through the functionalization of the surface of a dendrimer with a diamine motif, Kim et al. achieved the threading of CB[6] onto it, leading to a pseudorotaxinated surface architecture. ... 

A rotaxane composed of many ring parts is referred to as a poly[ ]rotaxane [6] these, in turn, include mostly main-chain and side-chain polyrotaxanes (Figure 36.1). In main-chain polyrotaxanes, the ring components are threaded along a polymeric axle, to form a polymer of [2]rotaxanes. A variety of polymers can be used as the long axle molecules, ranging from polymethylene chains to aromatic polymers. Among the different types of side-chain polyrotaxane can be included pseudo-polyrotaxanes and poly(2]rotaxanes. 

Main-chain-type polytotaxane Side-chain-type polyrotaxane... 

Figure 36.1 Main-chain polyrotaxane and side-chain polyrotaxane. Yellow = ring blue = axle red = stopper.

Polymers containing CDs as side chains have been prepared and reported [17], and shown to form inclusion complexes with both small and large molecules to produce pseudo-polyrotaxanes [90]. The first side-chain polyrotaxanes were prepared by Ritter et al. in 1995 [91], whereby the guest parts were immobilized in the CD cavities by capping the end groups using triphenyl groups. 

E and F). The essential difference in the main-chain structure between the two types of polyrotaxanes causes large differences in their physical or mechanical properties. Polyrotaxanes A-D are further divided into main- (A and B) and side-chain type (C and D). Meanwhile, the synthesis of poly[2]rotaxane (E) and poly[3]rotaxane (F) has been very recently achieved. Genuine poiyrotaxane seems to be one of the polyrotaxanes like the latter topological polyrotaxanes, which may reflect their truly unique structures to their properties. 

Scheme 5 Ritter s side-chain-type polyrotaxane synthesis.

One of the most promising methods to prepare side-chain-type polyrotaxane (C) is the polymer reaction of a certain reactive polymer with a pseudorotaxane. Ritter et al. prepared a polymethacrylate bearing carboxylic acid groups on the side chains as the reactive polymer. After the activation of the carboxylic acid with a chloroformate, aminolysis of the polymer with an amine-terminated CD-containing pseudorotaxane afforded the side-chain-type polyrotaxane (Scheme 5). " ... 

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